1. Field of the Invention
The present invention concerns a motor used as a driving means for such as a cooling fan being widely used in a household electric appliance or in a business electronic apparatus.
2. Description of the Prior Art
A motor of the prior art for driving a fan or the like comprises, in general, a stator assembly fixed to a motor frame, brackets each of which is mounted respectively to each of end portions of the motor frame, bearings each of which is mounted respectively to each of the brackets, a rotor axis rotatably supported by the bearings, and a rotor fixed on the external surface of the rotor axis. A fan such as a propeller fan or a sirocco fan for cooling is attached to an end portion of the rotor axis.
In the motor of the prior art as explained above, the rotor is caused to vibrate by electromagnetic force acting between the rotor and the stator assembly. The vibration of the rotor is transmitted to the fan attached to the end portion of the rotor axis through the rotor axis. Particularly, resonance of the vibration of the rotor with that of the fan causes a great noise. An example of such noise generated by a fan motor of the prior art is shown in FIG. 9. In the example shown in FIG. 9, the noise generated by the fan motor of the prior art shows its peak at about 1250 rpm of rotational speed of the fan.
In order to reduce noise of a fan motor caused by vibration of a motor, it is necessary to cut off, as much as possible, transmission of vibration of a rotor to a rotor axis and to a fan.
To reduce transmission of vibration of a rotor to a rotor axis, a motor in which the rotor is fastened to the rotor axis with an elastic body inserted between internal surface of the rotor and external surface of the rotor axis has been proposed.
In such motor in the prior art, however, there is a problem that the vibration is not sufficiently absorbed by the elastic body, and the transmission of the vibration can not be sufficiently reduced, when the elastic body is thin. On the other hand, if the elastic body is thick, there is another problem that air-gap between the stator assembly and the rotor is made to be fluctuated by electromagnetic force acting between the rotor and the stator assembly since fixing of the rotor to the rotor axis becomes insufficient, and the vibration can not be reduced.